1. Technical Field
The present invention relates to an electrophotographic image forming apparatus, and more particularly pertains to the control of an image forming apparatus in which multi-value image data is reproduced by pulse-width modulation of laser light.
2. Description of Related Art
There has been known an image forming apparatus that forms a two-dimensional (one-page) image by repetitively performing image formation for one or a plurality of lines in a first direction (main scanning direction) corresponding to image data, while driving an image carrier, such as a photoreceptor drum or a photoreceptor belt, in a second direction (sub-scanning direction).
By way of example, in an electrophotographic image forming apparatus, laser light modulated with image data is scanned in a main scanning direction, and concurrently therewith, an image is formed by the laser light on an image carrier (a photoreceptor drum or a photoreceptor belt), which is rotated in a sub-scanning direction.
Further, conventionally, in an image forming apparatus using laser light, use is made of a pulse width control system in which in order to represent multi-value image data including half tone as a toner image, the laser light used is pulse width modulated in correspondence to an image density (pixel data) of the image data.
In the above system, image data of each pixel is inputted to a pulse width modulator, which in turn outputs a pulse having a width corresponding to the image data. The pulse is received as an input at a laser driver in which the laser light is turned on and off at a timing of the pulse. In other words, a density gradation can be reproduced by adjusting, based on the image data, a width of the laser light scanned on a scanned object
Meanwhile, it is known that in the image forming apparatus such as described above, in which laser light is generated by use of a pulse width modulated pulse having a predetermined width, there is a problem in that depending on the performance of a laser driver or laser diode, laser light is not generated actually when the pulse width becomes narrower than a certain value.
FIG. 4 is a characteristic view illustrating an example of the relationship between PWM duty cycle and LD light emission amount. However, the characteristic is by way of example, and may become different depending on an environment such as characteristics of elements, pixel density or the like. Focusing on the relationship between the PWM duty cycle indicated on the horizontal axis (0%-100% (ratio that the pulse becomes on within the range of one pixel)) and the LD light emission amount indicated on the vertical axis (relative indication in 10 steps of 0-10, wherein 0 is non-light emission, and 10 is maximum light emission amount), it is seen that the LD light emission amount becomes 0 when the PWM duty cycle (pulse width) becomes smaller than or equal to 10% (d0 in FIG. 4).
In this way, when the LD light emission amount is not obtained, there is a problem in which a low density image cannot be reproduced faithfully. Therefore, countermeasures have been taken such as disclosed in Japanese Patent Laid-open Publication No. 2004-50789 (Patent Document 1) and Japanese Patent Laid-open Publication No. 2009-292058 (Patent Document 2).
In the above Patent Document 1, it is devised to allow LD to emit light even at a low density by making use of an increase of the pulse width due to contact (combination) between adjacent two pixels to cause light emission.
Further, in the above Patent Document 2, a technique is proposed to correct the pulse width in view of a difference between light emission time and non-light emission time when a line in a main scanning direction and a line in a sub-scanning direction are formed in an image forming apparatus in which light emission in the main scanning direction is repeated in the sub-scanning direction. For example, for the line in the main scanning direction, a state occurs in which the LD emits light substantially continuously, so that the LD is maintained in a state that facilitates light emission. Meanwhile, for the line in sub-scanning direction, a state occurs in which light emission is performed just for a moment during continuous non-light emission in the main scanning direction. Hence, for the line in the sub-scanning direction, a control such as facilitating light emission by increasing the pulse width is required.
As disclosed also in Patent Document 1, a technique exists to cause a pulse to be either left or right justified, in addition to causing the pulse to be located centrally within the range of one pixel, in order to enhance reproducibility of a thin line or make a diagonal edge appear smooth when image formation is performed by controlling laser light per pixel via pulse-width modulation.
FIGS. 12 and 13 illustrate, for two pixels adjacent to each other in the main scanning direction, pixel data, pixel position data, pulse width (PWM duty cycle) generated based on the pixel data, output pulse shape, and LD light emission status.
According to the characteristic view of FIG. 4, the LD light emission amount is 0 when the PWM duty cycle is 10%. However, as in FIG. 12, when a right justified pixel and a left justified pixel contact with each other and the total PWM duty cycle is 20% (10%+10% in FIG. 12(a), FIG. 12(b)), an LD light emission amount according to the PWM duty cycle of 20% is generated (FIG. 12(c)).
However, when three pixels, i.e., aright justified pixel, a center justified pixel and left justified pixel exist independently without contacting with each other as in FIG. 13, the LD light emission amount remains zero (FIG. 13(c): non-light emission) even if the PWM duty cycle is 10%+10%+10% (FIG. 13 (b)).
In other words, it has been discovered by the present inventor that a reversal phenomenon (tone jump) occurs for the relationship between the pixel data and the light emission amount as in cases in which light emission is performed with pixel data of 32+32 (refer to FIG. 12) and in which light emission is not performed even with pixel data of 32+32+32 (refer to FIG. 13).
Meanwhile, there is also a technique that does not make use of a low density region in which such tone jump occurs. However, in the recent image forming apparatus, a high resolution has been achieved, and the pulse width of one pixel has become narrower than the conventional one. Therefore, it is undesirable that there is a region which cannot be used or a region in which a tone jump occurs. Further, also when achieving a high gradation in an image forming apparatus, it is undesirable that there is a region which cannot be used or a region in which a tone jump occurs.